1. Field of the Invention
This invention relates to electron guns for use with traveling wave tubes and, more particularly, to the grid structure of such electron guns as are focused by means of periodic-permanent-magnets.
2. Description of the Prior Art
A traveling wave tube (TWT) is a device for bringing together within the same space an electron beam and radio-frequency (rf) energy for interaction with one another to produce a desired effect. The rf energy is provided from an external source to the rf input port of the TWT. The electron beam is provided from an electron gun. A traveling wave tube (TWT) typically comprises a generally cylindrical body having a hollow axially-extending, cylindrical electron beam tube. A series of axially-extending cavities are radially positioned around the beam tube, each of the cavities having an aperture which connects or couples that cavity to the beam tube. These series of cavities are generally referred to as the circuit of the TWT. A magnetic focusing means, which is also radially positioned around the TWT circuit, produces a magnetic field, the axis of which coincides with the axis of the beam tube. The magnetic focusing means is generally either a solenoid type or a periodic-permanent-magnet (PPM) type. The magnetic field functions to exert a compressive force on the electron beam to focus it and confine it to flow within the electron beam tube in a laminar and uniform manner.
TWTs are frequently used in applications which require the TWT to be capable of alternately operating at a low power level and a high power level. The high and low power levels refer to the power level of the output power. Typically, the applications call for the high power level to require an electron beam current five times the electron beam current of the low power mode. The beam current is determined by the amount of electrons emitted from the cathode of the electron gun. To control the amount of electrons emitted from the cathode surface, electron gun grids are used. An illustration of the use of electron gun grids may be found in the device described in U.S. Pat. No. 3,812,395 issued to Scott. The electron gun is typically a Piercegun as described in "Theory and Design of Electron Beams" by J. R. Pierce, D. Van Nostrand Company, Inc., 1954.
In the device disclosed by Scott, the voltage on an electron gun control grid is varied to alternately provide a high beam current during the high power mode and a low beam current during the low power mode. However, the decreased voltage on the control grid during the low power mode causes a corresponding decrease in the amount of electrons emitted from the cathode. The decrease in the number of electrons reduces the beam space charge density. The reduced beam space charge density is thus less effective at counter-balancing the radially-inward compressive force of the magnetic field and, as a result, the electron beam collapses or defocuses from the focused, laminar state of the high power mode. Theoretically, the available low beam current should be one fifth of the amount of the available high beam current if the beam current emitted at the cathode is decreased by a factor of five. However, the low beam current that is actually available in the TWT is less because of the collapse of the beam. The collapse of the beam causes the beam to become defocused, with the result that a part of the beam intercepts the wall of the electron beam tube. For example, in a PPM-focused TWT, only approximately 50% of the theoretical amount of the low power beam current is actually transmitted through the beam tube (as compared to approximately 90% for a solenoid-focused TWT). In contrast, the transmission percentage during the high power mode is approximately 92% of the theoretical for the PPM type and 98% for the solenoid-focused type.
To alleviate the collapsing beam problem, control grids are designed and used to maintain the space charge density constant in the electron beam during the low power mode, that is, to maintain the low power mode space charge density at the level of the high power mode space charge density. This is generally accomplished by biasing the voltage on a portion of a control grid to cause a corresponding portion of the cathode electron emitting surface to be non-emitting. This produces a smaller diameter electron beam during the low power mode. The smaller diameter beam, which has a space charge density equal to that of the beam of the high power mode, does not collapse. Such a prior art control grid operation is disclosed in U.S. Pat. No. 4,023,061, issued to Berwick et al. However, this solution gives rise to problems of its own.
The control grid, of a device as disclosed in Berwick, has been biased to permit only a small diameter beam to be emitted from the cathode during the low power mode of operation. This effects a space charge density in the low power mode equal to the space charge density of the high power beam. However, the now narrow low power mode electron beam interacts poorly with the radio-frequency signal that is present at the cavity apertures of the TWT circuit. The radio-frequency signal, propagating through the cavities, creates an axially-extending cylindrical electric field that is also concentric with the hollow cylindrical beam tube. In order to amplify the radio-frequency signal, electrons in the beam tube must interact with the axial electric field. However, the electric field, due to its inherent properties, generally concentrates at the inner peripheral surface of the electron beam tube adjacent the cavity apertures. Thus, the narrow beam, which has a diameter that is much smaller than the diameter of the beam tube, interacts inefficiently with the radio-frequency signal. The efficiency of interaction, generally referred to as the electronic efficiency, is generally decreased by as much as 50% during the low power mode for the grid structures similar to that of the device shown in Berwick.
A second disadvantage of devices similar to that of Berwick results from the physical structure of the grid assembly. The control grid is comprised of two concentric grids. The radially inner grid is smaller and controls the low power mode. The radially outer grid is annular and circumscribes the radially inner grid. Together, the two grids cover a larger area and control the high power mode. Because the two grids are physically and electrically insulated from one another, the support structure of the radially inner grid must traverse the flow of emitted electrons during the high power operating mode. The support structure will thus intercept electrons in the high power mode and cause distortions in the electron optics of the high power mode.
Any prior art devices which have a low power beam of a diameter significantly less than the diameter of the high power beam will suffer from reduced electronic efficiency in the low power mode. In addition, any electron gun grid structure which has a first control grid supported concentrically with respect to a second control grid will suffer distortions in the electron optics when the beam operates in the mode wherein the radially outer control grid is activated.